(a) Field of the Invention
The present invention relates to an apparatus and method for processing ranging channels in an orthogonal frequency division multiple access (OFDMA) system. More specifically, the present invention relates to an apparatus and method for processing ranging channels to measure the propagation delay and the power of each mobile station on the reverse link of an OFDMA system.
(b) Description of the Related Art
For realization of a wireless wideband multimedia system that provides large-capacity services with high reliability, the OFDM system has been spotlighted in that it can transmit signals at a high transmission rate through time division multiple access (TDMA) wireless channels in a millimeter wave band of several to several tens of GHz.
The OFDM system enhances the frequency use efficiency by using subcarriers having mutual orthogonality, and overcomes the characteristic of signals for multi-path channels with a one-tap equalizer because the period of data signals changes slowly in that system. OFDM, in which the digital signal processing part is realized rapidly by fast Fourier transform (FFT), is also widely used for high-speed communication systems. This system is used, for example, in wireless communication systems such as digital audio broadcasting (DAB), digital video broadcasting (DVB), IEEE 802.11a, HIPERLAN/2, and so forth.
The orthogonal frequency division multiple access (OFDMA) system, which is a multiple access system designed for simultaneous access of multiple users, is applied to OFDM. OFDMA divides an allocated frequency band into N subcarriers and allocates the subcarriers into groups for simultaneous use by multiple links.
On the forward link from a base station to a plurality of mobile stations, the subcarrier groups allocated to the respective mobile stations are transferred simultaneously while they are in time synchronization with one another, thereby guaranteeing mutual orthogonality of the subcarriers. On the reverse link from a plurality of mobile stations to the base station, however, signals randomly transferred from the multiple mobile stations arrive at the base station with different time delays and different power, and a loss of signal orthogonality occurs when the time delays and the power exceed a protection interval and a power level allowed to the base station, thus causing a serious loss of signals.
In an attempt to control the time delays and power properties of the random signals of the mobile stations, the OFDMA system may employ a ranging system that allocates a subcarrier group to a defined ranging channel and controls the base station to measure propagation delay and power using the ranging channel received from each mobile station.
The mobile station transmitter and the base station receiver using the conventional ranging system are described as follows. For initial connection and data transmission, the mobile stations randomly select signals of a predetermined pseudo noise code and convert them using a subcarrier group allocated to the ranging channel. The ranging channel is converted to a time-domain signal through inverse fast Fourier transform (IFFT). For protection from inter-symbol interference (ISI), the time-domain signal is combined with a cyclic prefix (CP) into a baseband signal. The baseband signal thus generated is transmitted on a wireless channel by RF signal processing and sent to the base station receiver.
Upon receiving signals with different delays from the mobile stations simultaneously, the base station receives baseband signals by RF signal processing and converts them to frequency-domain signals by fast Fourier transform (FFT). The ranging channel is selected from the frequency-domain signals to measure the correlations of all the pseudo noise codes and time delays.
In the description of the ranging channel processing procedure, it is assumed that S time delays are predetermined in the system. Each of the received ranging complex signals R(k) having P lengths is multiplied by a phase rotation component exp(−j2πkn0/N) corresponding to a specific delay n0 to eliminate the phase component from the signal for the specific time delay. Each of the signals removed of the phase component is correlated with a set of Q ranging codes. The correlation value thus determined is compared with a threshold, and the power is calculated when the correlation value is greater than the threshold. The correlation with a set of Q ranging codes is similar to general CDMA pseudo noise code demodulation, and includes a multiplication of the corresponding code by the received signals to calculate the sum of the signals. In this way, S arithmetic operations of as many as the number of time delays are performed. When the signal is greater than the threshold, the corresponding delay and the code and power are reported to the upper system.
The propagation delay of each mobile station to be measured by the base station is proportional to the round-trip propagation delay between the base station and the mobile station and the RF processing delay, and the number of time delays to be measured increases with an increase in the distance between the base station and the mobile station. In the case of using complex coordinates, the complexity for a specific time delay is given by the multiplication of the received complex signal by the phase rotation complex signal, and P×Q×4 multiplications and P×Q×2 additions are needed. Accordingly, in the receiver structure of the ranging method, the complexity of H/W increases with an increase in the number of pseudo noise codes or the timing for the time delay to be measured, so it takes an excessive amount of time to measure the propagation delay time and the power of the mobile stations with a corresponding deterioration of efficiency.